Oligomers of olefin are used for various purposes and, particularly, dimers of low-molecular-weight olefin (e.g., propylene, n-butene, isobutene, pentene, etc.) are important as high-octane number base materials for gasoline production, or important as chemical intermediate raw materials. Oligomerization including olefin dimerization is carried out using acid catalysts, and many studies have been made therefor. Conventional examples of the acid catalyst include sulfuric acid, hydrogen fluoride, phosphoric acid, aluminum chloride, boron fluoride, amorphous or crystalline aluminosilicate, clay, ion-exchange resin, mixed oxide, and solid acid such as an acid supported on solid carrier, and various examinations have been made also for the solid phosphoric acid catalyst which provide inexpensive and simplified chemical manufacturing processes.
There are disclosed to oligomerize propylene using a solid phosphoric acid prepared in a calcination condition of 100° C. or higher (Patent Literature 1) and to oligomerize propylene using a catalyst prepared by crystallizing an amorphous mixture of phosphoric acid and a siliceous raw material in conditions of 250-450° C. and water vapor concentration of 3-50 mol % (a catalyst consisting of silicon orthophosphate and silicon pyrophosphate) (Patent Literature 2).
It is hitherto known that the condensation degree of phosphoric acid in the solid phosphoric acid catalyst affects the activity of oligomerization reaction of olefin, and there also are disclosed examples of oligomerization of olefins of C3, C4 and the like using a catalyst in which the mass ratio of a free phosphoric acid component which is eluted when the solid phosphoric acid catalyst is dipped in water (non-condensed or low-condensed phosphoric acid such as orthophosphoric acid or pyrophosphoric acid) to the catalyst is small (the proportion of orthophosphoric acid in the phosphoric acid supported is about 46 mol % at most in terms of phosphorus atom amount) (Patent Literature 3 and Non-Patent Literature 1).
However, not only the oligomerization of olefin using the above-mentioned conventional phosphoric acid catalysts is not mainly intended for dimerization of olefin, but also the use of the conventional solid phosphoric acid catalysts unavoidably involves by-production of high polymerized products of olefin, and it was thus difficult to selectively provide dimers of olefin.
In a phosphoric acid aqueous solution supporting operation, though affected by operating condition such as operating time or temperature, condensation of the supported orthophosphoric acid in drying process is easy to proceed. Even after once supported, the condensation easily proceeds in the same manner. As described later, particularly, a low orthophosphoric acid concentration in the total phosphoric acid facilitates the proceed of the condensation. As described in Patent Literature 3, also, a highly-condensed phosphoric acid which is once condensed is dissolved only very slowly to water (Patent Literature 3, the latter part of Paragraph 0036). Namely, the progress of hydrolysis of the highly-condensed phosphoric acid is virtually slow.
In other words, in the conventional phosphoric acid aqueous solution supporting operation, the condensation is consequently easy to proceed due to the low orthophosphoric acid concentration in the total phosphoric acid or the like, and the supported amount of highly-condensed phosphoric acid must be increased. Therefore, in the above-mentioned case (the proportion of orthophosphoric acid in the phosphoric acid supported being about 46 mol % at most in terms of phosphorus atom amount), the actual proportion of orthophosphoric acid is predicted to be far less than the maximum value as a theoretic value (about 46 mol %) because the condensation progresses even if the orthophosphoric acid is produced. It is not easy for the highly-condensed phosphoric acid which is once condensed and supported in the drying process to return to orthophosphoric acid even if it is thereafter hydrolyzed by contact with liquefied or gaseous water in a reaction system. Further, in the supported orthophosphoric acid, the condensation of orthophosphoric acid can proceed depending on the reaction condition even if the orthophosphoric acid concentration in the total phosphoric acid is high, as well as being low.    Patent Literature 1: Japanese Patent Examined Publication No. 8-29251    Patent Literature 2: Japanese Patent Examined Publication No. 7-59301    Patent Literature 3: Japanese Patent Laid Open No. 2001-199907    Non-Patent Literature 1: “Applied Catalysis A: General”, 1993, 97, p. 177-196